SPD light valves have been known for more than seventy years for use in the modulation of light. Such light valves have been proposed for use in numerous applications during that time including, e.g., alphanumeric displays and television displays; filters for lamps, cameras, displays and optical fibers; and windows, sunroofs, toys, sunvisors, eyeglasses, goggles, mirrors, light pipes and the like to control the amount of light passing therethrough or reflected therefrom as the case may be. Examples of windows include, without limitation, architectural windows for commercial buildings, greenhouses and residences, windows, visors and sunroofs for automotive vehicles, boats, trains, planes and spacecraft, windows for doors including peepholes, and windows for appliances such as ovens and refrigerators including compartments thereof. Light valves of the type described herein are also known, as indicated above, as suspended particle devices or SPDs.
As used herein, the term “light valve” describes a cell formed of two walls that are spaced apart by a small distance, with at least one wall being transparent. The walls have electrodes thereon, usually in the form of transparent, electrically conductive coatings. The coatings are commonly, but not necessarily, formed of indium tin oxide. The electrically conductive coatings can be deposited on the walls in patterns so that different segments of the light valve can be selectively activated. Additionally the electrodes on the walls may have thin transparent dielectric overcoatings thereon. The cell contains a light-modulating element (sometimes herein referred to as an activatable material) which may, without limitation, be either a liquid suspension of particles, or all or a portion of the entire element may comprise a plastic film in which droplets of a liquid suspension of particles are distributed.
The liquid suspension (sometimes herein referred to as a liquid light valve suspension or as a light valve suspension) comprises small particles suspended in a liquid suspending medium. In the absence of an applied electrical field, the particles in the liquid suspension assume random positions due to Brownian movement. Hence, a beam of light passing into the cell is reflected, transmitted or absorbed depending upon the cell structure, the nature and concentration of the particles and the energy content of the light. The light valve is thus relatively dark in the OFF state. However, when an electric field is applied through the liquid light valve suspension in the light valve, the particles become aligned and for many suspensions most of the light can pass through the cell. The light valve is thus relatively transparent in the ON state. The ΔT is defined as the difference in visible light transmission between the ON and OFF states.
For many applications it is preferable for all or part of the activatable material, i.e., the light modulating element, to be a plastic film rather than a liquid suspension. For example, in a light valve used as a variable light transmission window, a plastic film in which droplets of liquid suspension are distributed is preferable to a liquid suspension alone because hydrostatic pressure effects, e.g., bulging associated with a high column of light suspension, can be avoided through use of a film and the risk of possible leakage can also be avoided. Another advantage of using a plastic film is that, in a plastic film, the particles are generally present only within very small droplets and, hence, do not noticeably agglomerate when the film is repeatedly activated with a voltage.
A light valve film (also sometimes herein referred to as an SPD film) as used herein means a film or sheet, or more than one thereof comprising a suspension of particles used or intended for use in an SPD light valve. Such light valve film usually comprises a discontinuous droplet phase of a liquid or liquids comprising dispersed particles (liquid light valve suspension), such discontinuous phase being dispersed throughout a solid continuous matrix phase, said phases enclosed within one or more rigid or flexible solid films or sheets. The combined aforesaid phases are referred to as the cured SPD emulsion, which may be part of a light valve film, sometimes also referred to as a film or film layer. The light valve film and/or laminate of the light valve film may also comprise one or more additional layers such as, without limitation, a film, coating or sheet or combination thereof, which may provide the light valve film with one or more of, for example, (1) scratch resistance, (2) protection from ultraviolet radiation, (3) reflection of infrared energy, (4) electrical conductivity for transmitting an applied electric or magnetic field to the activatable material, (5) dielectric overcoatings, i.e., for providing insulation protection against short circuits, (6) color tinting and (7) acoustic control.
A common (but non-limiting) construction for an SPD film has five layers, namely, from one side to the other: (1) a first sheet of polyethylene terephthalate (“PET”) plastic, conveniently 5-7 mils in thickness, (2) a very thin transparent, electrically conductive coating (typically of indium tin oxide (“ITO”)), acting or capable of acting as an electrode, on said first sheet of PET, (3) a layer of cured (i.e., cross-linked) SPD emulsion, usually 2-5 mils in thickness and, (4) a second coating (typically ITO) acting or capable of acting as an electrode on (5) a second PET plastic substrate. As stated previously, additional layers which provide other functions may optionally be added to the five-layer SPD film described above. Typically, copper foil, conductive fabric or the like are affixed to the electrodes so that they extend beyond the perimeter of the SPD film for convenient connection to a suitable voltage source. Furthermore, the SPD film can be laminated (see U.S. Pat. No. 7,361,252 assigned to the assignee of the present invention), for example, between transparent hot melt adhesive films and/or glass or thicker transparent plastic sheets to provide strength and rigidity and to protect various parts of the combined unit from environmental stresses which may, otherwise, damage its performance characteristics.
Electric power to actuate the light valve and film can be from any conventional or non-conventional source. For example, the assignee of the present invention has publicly demonstrated operation of an SPD film and light valve powered by photoelectric/photovoltaic energy that may be derived from solar energy or an alternative light source such as a lamp.
U.S. Pat. No. 5,409,734 exemplifies a type of non-cross-linked light valve film that is made by phase separation from a homogeneous solution. Light valve films made by cross-linking (curing) of emulsions are also known. The methods of the present invention are specifically directed to the use of the latter type of film, i.e., film comprising a layer formed by cross-linking an emulsion, and to laminated films produced thereby. See, for example, U.S. Pat. Nos. 5,463,491 and 5,463,492, and U.S. Pat. No. 7,361,252, all of which are assigned to the assignee of the present invention. Various types of SPD emulsions, and methods of curing same, are described in U.S. Pat. Nos. 6,301,040, 6,416,827, and 6,900,923 B2, all of which are assigned to the assignee of the present invention. Such films and variations thereof may be cured through cross-linking brought about by exposing the films to (1) ultraviolet radiation, (2) electron beams or (3) heat. All of the patents and patent applications and other references cited in this application are incorporated herein by reference.
A variety of liquid light valve suspensions are well known in the art and such suspensions are readily formulated according to techniques well-known to one of ordinary skill therein. The term liquid light valve suspension, as noted above, when used herein means a liquid suspending medium in which a plurality of small particles are dispersed. The liquid suspending medium comprises one or more non-aqueous, electrically resistive liquids in which there is preferably dissolved at least one type of polymeric stabilizer that acts to reduce the tendency of the particles to agglomerate and to keep them dispersed and in suspension.
Liquid light valve suspensions useful in the present invention may include any of the so-called prior art liquid suspending media previously proposed for use in light valves for suspending the particles. Liquid suspending media known in the art which are useful herein include, but are not limited to, the liquid suspending media disclosed in U.S. Pat. Nos. 4,247,175, 4,407,565, 4,772,103, 5,409,734, 5,461,506, 5,463,492, and 6,936,193 B2, the disclosures of which are incorporated herein by reference. In general one or both of the suspending medium or the polymeric stabilizer typically dissolved therein is chosen so as to maintain the suspended particles in gravitational equilibrium.
The polymeric stabilizer, when employed, can be a single type of solid polymer that bonds to the surface of the particles, but which also dissolves in the non-aqueous liquid(s) which comprise the liquid suspending medium. Alternatively, there may be two or more solid polymeric stabilizers serving as a polymeric stabilizer system. For example, the particles can be coated with a first type of solid polymeric stabilizer such as nitrocellulose which, in effect, when dissolved, provides a plain surface coating for the particles, together with one or more additional types of solid polymeric stabilizer that when dissolved, bond to or associate with the first type of solid polymeric stabilizer and also dissolve in the liquid suspending medium to provide dispersion and steric protection for the particles. Also, liquid polymeric stabilizers may be used to advantage, especially in SPD light valve films, as described for example in U.S. Pat. No. 5,463,492.
Inorganic and organic particles may be used in a light valve suspension, and such particles may be either light absorbing or light reflecting in the visible portion of the electromagnetic spectrum.
Conventional SPD light valves have generally employed particles of colloidal size. As used herein the term colloidal means that the particles generally have a largest dimension averaging 1 micron or less. Preferably, most polyhalide or non-polyhalide types of particles used or intended for use in an SPD light valve suspension will have a largest dimension which averages 0.3 micron or less and more preferably averages less than one-half of the wavelength of blue light, i.e., less than 2000 Angstroms, to keep light scatter extremely low.